Can viewport playback be improved by moving elements to other layers?
Blender viewport performance is known to degrade with many objects, and frame rate can decrease significantly depending on scene complexity.

The rendering system is a bit archaic and doesn't currently take full advantage of high end hardware, it's planed to be overhauled in the near future, but unfortunately there is not much one can currently do to to overcome the underlying problem until then.

Where your images are loaded from (external hard drive, local disk, or packed in blend file) as little effect on viewport performance, except when reading them for the first time, before they are loaded into memory.

That being said, hiding objects that are not needed has a huge influence in viewport performance, you can either hide unneeded objects, move them to different layers, or even organized them into several distinct groups and use the Outliner to toggle viewport visibility en-mass. All those should have a positive impact in performance.

Additionally depending on what render engine you are using, (if you are in Blender Internal or Game Engine) you can switch to GLSL viewport shading mode which is said to have slightly better performance. N Properties Panel > Shading > GLSL

If you have an nVidia graphics card you can also try to turn on VBOs in the user preferences, which generally give quite a considerable performance bump in many cases. You can find it in User Preferences > System Tab > OpenGL Column > VBOs Checkbox.